1. Field of the Invention
This invention relates to an external combustion engine for converting the displacement of a liquid portion of a working fluid caused by a volume change of the working fluid due to the generation and liquefaction of the vapor of the working fluid into mechanical energy and then outputting the mechanical energy.
2. Description of the Related Art
One type of external combustion engine is known to have the configuration in which a working fluid in a liquid state is sealed in a pipe-like container, the vapor of the working fluid is generated by heating part of the working fluid in the container by a heater, the vapor of the working fluid is cooled and liquefied by a cooler to thereby change the volume of the whole working fluid, and the displacement of the liquid portion of the working fluid generated by the volume change of the working fluid is converted into mechanical energy and then outputted (See, for example, Japanese Unexamined Patent Publication No. 2005-330910).
In the above technique, the heater is arranged above the cooler, and when part of the working fluid is heated by the heater, the high-temperature high-pressure vapor of the working fluid is accumulated on the portion of the container where the heater is arranged, so that the liquid level of the working fluid is pushed down toward the cooler. As a result, the liquid portion of the working fluid is displaced downward in the container.
The vapor of the working fluid, advancing into the portion of the container where the cooler is arranged, is cooled and liquefied by the cooler. Therefore, the force to push down the liquid level of the working fluid is lost, and the liquid level of the working fluid rises into the heater, and the liquid portion of the working fluid is displaced upward. By repeating this operation, the liquid portion of the working fluid is moved and displaced periodically. In the process, the internal pressure of the container changes periodically.
Japanese Patent Application No. 2006-78802 (hereinafter referred to as the prior application) proposes an external combustion engine improved in output and efficiency. This prior application is intended to improve the output and efficiency of the external combustion engine by controlling the average value of the internal pressure of a container toward a target value.
More specifically, the working fluid in a liquid state is sealed in an auxiliary container separate from a main container sealed with the working fluid, the main container and the auxiliary container communicate with each other through a choke, and the working fluid in the auxiliary container is compressed or expanded by a piston mechanism thereby to control the internal pressure of the auxiliary container.
In this configuration, since the main container and the auxiliary container communicate with each other through the choke, the internal pressure of the auxiliary container does not change periodically with the internal pressure of the main container, and can be stabilized at a level substantially equal to the average value of the internal pressure of the main container. Thus, a target value of the internal pressure of the main container is calculated based on the temperature of a heater, etc., and the internal pressure of the auxiliary container is controlled toward the target value by a piston mechanism. As a result, the average value of the internal pressure of the main container near the target value can be obtained.
According to the prior application described above, in the case where the external combustion engine stops and the heater stops heating the working fluid, the temperature of the heater gradually drops to ambient temperature. As long as the vapor of the working fluid is accumulated in the main container when the external combustion engine stops; however, the saturated vapor pressure of the working fluid also drops with the heater temperature, resulting in condensation and liquefaction of the vapor of the working fluid. Thus, the internal pressure of the main container drops.
Once the internal pressure of the main container drops below the internal pressure of the auxiliary container, the working fluid in the auxiliary container gradually begins to flow into the main container through the choke, and the volume of the working fluid in the main container increases excessively. This phenomenon is more likely to occur in winter when the ambient temperature is low.
In the case where the external combustion engine is restarted and the working fluid is heated by the heater with an excessive volume of the working fluid in the main container as described above, part of the working fluid is gasified and the internal pressure of the main container rises. Once the internal pressure of the main container increases beyond the internal pressure of the auxiliary container, the excess working fluid in the main container is returned to the auxiliary container through the choke.
Since only a small amount of the working fluid can flow through the choke at a time, considerable time is required before all of the excess working fluid in the main container returns to the auxiliary container. As a result, a predetermined output cannot be produced, before all of the excess working fluid in the main container can return to the auxiliary container after the engine restarts, thereby posing the problem that the restarting time is lengthened before the predetermined output is obtained.
In order to avoid the above engine restart problem, the external combustion engine is required to be stopped at a when the vapor of the working fluid is not accumulated in the main container, thereby greatly complicating the operation to stop the external combustion engine.